<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            font-size: 10px;
        }

        .box {
            width: 100rem;
            height: 200px;
            background-color: pink;
        }

        @media screen and (max-width: 768px) {
            html {
                font-size: 6px;
            }
        }

        /* @media screen and (max-width: 768px) {
      .box {
        width: 500px;
      }
    }
    @media screen and (max-width: 480px) {
      .box {
        width: 300px;
      }
    } */
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>